1. Technical Field
The embodiments herein generally relate to wireless communication systems, and, more particularly, to a fast fading channel Fast Fourier Transform (FFT) trigger point tracking in Integrated Services Digital Broadcasting (ISDB) receivers.
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM) is a digital data modulating technique, which uses a large number of closely spaced orthogonal sub-carriers. The digital data is modulated to an amplitude and a phase of each of the orthogonal sub-carriers within a transmission band. In a digital broadcasting receiver design of ISDB receivers such as Integrated Services Digital Broadcasting-terrestrial (ISDB-T) and Integrated Services Digital Broadcasting-terrestrial digital sound broadcasting (ISDB-TSB) for OFDM systems, a major challenge lies in FFT trigger point tracking.
Terrestrial digital broadcasting using an OFDM method is susceptible to inter-symbol interference of multiple sub-carrier waves; the signal obtained is a composite wave resulting from the combination of the multiple sub-carrier waves received by an OFDM receiver. This causes fading (e.g., distortion in a carrier-modulated signal) of the transmitted OFDM symbols. The inter-symbol interference may be avoided by a FFT computation. In an OFDM receiver, the FFT computation such as trigger point tracking is performed by an FFT computing circuit in the receiver, by which the received OFDM signal is demodulated.
Trigger point is a point at which the sampling of an input signal starts. The traditional method for FFT trigger point tracking is based on a channel impulse response of the received OFDM signal. The channel impulse response refers to an output signal (an infinitely high peak) obtained for an input signal in a communication channel. The channel impulse response is usually obtained by performing an inverse FFT (IFFT) of the time-domain interpolated channel estimates or performing an IFFT on the scatter pilots.
For fast fading channel as in OFDM systems, performing an IFFT of the time-domain interpolated channel estimates or on the scatter pilots typically leads to degradation, noisy channel estimates, a short channel impulse response (e.g., due to limited scatter pilot spacing), aliasing (e.g., distortion of a frequency in a signal), and an incorrect trigger point adjustment.
For example, two trigger points which are Tu/12 (e.g., Tu is the useful OFDM symbol time in ISDB-T and ISDB-TSB) apart have the same channel impulse response and generally cannot be distinguished. This poses a problem while interpolating the channel in the frequency domain. Hence, the traditional method of performing an IFFT based on the channel impulse response in the frequency domain is generally not successful in FFT trigger point tracking.